1. Field of the Invention
The present invention relates to a gas turbine.
2. Description of Related Art
A gas turbine is equipped with a compressor, a combustor, and a turbine. In the gas turbine, air is compressed in the compressor and flows into the combustor where it is mixed with fuel and combustion occurs. The combustion gas flows into the turbine where energy is extracted from the gas to rotate the compressor and to drive a generator to generate electricity. After flowing through the turbine, the combustion gas is exhausted through an exhaust diffuser.
FIG. 4 shows an example of a turbine equipped with an exhaust diffuser. The turbine consists of multiple stationary airfoils (vanes, not shown) attached to outer casing 3, and multiple rotating airfoils 2 (blades) which are attached to rotor shaft 1, which rotates about centerline CL. The gas flow, F, is in the direction or left to right on FIG. 4. The turbine can consist of multiple pairs of vanes and blades (stages) attached to rotor 1. FIG. 4 shows the blade of the last stage of the turbine.
The exhaust diffuser, consisting of parts 5, 6, 7, and 8 is connected coaxially to the downstream end of the turbine. The exhaust diffuser consists of exhaust casing 6 which encases gasflow path 5 and multiple struts 8 which support journal bearing 7 which in turn supports rotor 1.
Each strut 8 is equipped with strut main body 8a, that supports journal bearing 7, and strut cover 8b that covers and protects strut main body 8a from the combustion gas F.
In the above conventional gas turbine, strong shock waves can form at the leading edge of each strut cover 8b, resulting in reduced turbine performance. FIG. 5 shows the conventional cross section A-A of strut 8. The shape of strut cover 8b consists of parallel lines in the flow direction connected by semicircles at the leading edge LE and trailing edge TE.
As the combustion gas F, having high Mach number (for example, M=0.65), flows over the strut leading edge, the flow speed rapidly increases to achieve supersonic speed. A shock is generated in the regions indicated by “a” of FIG. 5. The presence of the shock has the effect of reducing turbine efficiency.
This effect on turbine efficiency is increased when the ambient temperature (temperature at the compressor inlet) is low. The amount of air flowing into the gas turbine at low ambient temperature is larger than that at normal ambient temperature, and as a result, the Mach number of the combustion gas flowing into the exhaust diffuser is increased. Accordingly, the shock wave generated at the leading edge LE becomes stronger, resulting in further reductions in turbine efficiency.